Hi again, I just found https://cdn.head-fi.org/a/7514507.jpg and looks like the 6K resistor connected to pin-2 of the opamp gets to capacitor's + (plus) and the resistor coming from pin-6 gets to - (minus) of the other capacitor. I wonder if this is not the Ground? 'Cause if it's the ground there, then based on http://www.ti.com/lit/ds/symlink/lme49860.pdf we should have 2 resistors there...though their values are different, but that could be Matrix's choice, of course. I believe these 2 resistors are the ones taking care of amplifier's gain (along with R7 and R9 I assume, but can't be sure without having the schematic); more details could also be found here: http://www.wakamatsu-net.com/biz/snas435b.pdf (or just read the LME49610 datasheet). In case someone modified the default internal fixed gain ratio of the amplifier, then all the 3 adjustable gains have been modified too. If I'm not mistaken, the default gain selector for HPA-3U are below: 5dB = 1.75X (3.56V RMS output with 2V input) 10dB = 3.15X (6.13V RMS output with 2V input) 20dB = 10X (20.0V RMS output with 2V input) If you have another headphone amplifier with a well-known gain, then you could A/B test your HPA-3U against the other amplifier and you could approximate if HPA-3U's gain is the same as Matrix intended. Also, measuring RMS output voltage with a 1KHz 2V RMS input sine-wave (audiocheck.net perhaps) could give you a good idea about the internal gain of your amplifier. A scope would be perfect for this, but also a true-RMS volt-meter will do to. Good luck! P.S.: I'm pretty sure those resistors should not change sound quality of how the HPA-3U sounds, so you could just ignore them and enjoy the sound.